/**
 * @param {string} s
 * @param {string} t
 * @return {number}
 */
 var appendCharacters = function(s, t) {
     let left = 0;
     for(let i = 0; i < s.length; i++){
         if(s[i] == t[left]) left++;
     }
     return t.length - left;
};